The sound of your cajon is very adjustable. There's a bass tone in the middle area, and on the edge there's a slap or a crack. With this guide, you will learn how to adjust the corner crack tone.
Start with a far corner of the faceplate - furthest from the body of the cajon. When the plate is less taught, the drum gives a slow dynamic and a rather pronounced crack. That sound might be too much for some people.
To adjust the response, take a Phillips Head screwdriver and very gently start to close that gap by tightening the screw. As the gap tightens, the response time quickens and the crackle effect is reduced.
If you want it to go away altogether, make the plate good and snug. Do not over-tighten. With the constant contact there will be no crackle effect.
Now, work the opposite corner to create a satisfying dynamic when you play your cajon drum with both hands.
If the opposite corner is tight already, put the screwdriver in and give it about a half of a turn counterclockwise. That's should be all it takes to create a variance in tone.